Prior art optical-electronic measuring methods for the testing or measuring of parts or workpieces, utilize the shadow produced by the workpiece, when the workpiece is illuminated with a beam of radiation. The shadow is detected by sensing apparatus which produces electrical signals that are interpreted for purposes of measurement. Thus, these prior art measuring methods use geometrical optics as opposed to wave optics.
The degree of measuring precision that can be achieved by these measuring methods often is not sufficient. In addition, when using such measuring methods, it is often required that precise distances be maintained between light source, workpiece and sensor. There may be an unfavorable effect as concerns measuring precision, if the distances cannot be maintained with the required precision during construction or assembly, or if the size of the workpiece or object to be measured is changed during the measuring process, for instance due to temperature changes or vibrations. In the prior art measuring methods, it is often necessary that the required precision be maintained even upon a displacement of the measuring instrument longitudinally relative to the workpiece to be measured. Another disadvantage is that to obtain a precise measurement the sensor must have a high capacity of resolution. It should be noted that many sensors which are available at the present time do not have a satisfactory resolution capacity. This problem may be solved in part when the electronic signals produced by the sensor are evaluated, which, however, detracts from the reliability of the measuring results.
German patent application DE-OS No. 2458807 discloses a method for measuring the distance between parallel lines in connection with the manufacture of semiconductor integrated circuits. The method of the German patent application uses diffraction patterns. Here the diffraction pattern is obtained from light reflected by the object measured. Because of the required reflection of the light by the object measured, application of this measuring method is very limited and can only be used in connection with devices such as semiconductor integrated circuits.
It is the object of the present invention to create a method and apparatus for the optical electronic measuring or testing of a workpiece or other objects wherein the disadvantages inherent in the known methods can be prevented. It is a further object of the invention to provide a measuring method and apparatus that has a measuring precision in the micron range and has multi-application purposes, such as the measuring of diameters and longitudinal distances as well as for testing or determining the contours of parts.